We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4À3658, an X-ray transient with a recurrence time of %2 yr, using data from the Rossi X-Ray Timing Explorer covering four transient outbursts (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray ''hot spot'' on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous˙measurements of this source into question), with typical upper limits of j˙j P 2:5 ; 10 À14 Hz s À1 (2 ). However, combining data from all the outbursts shows with high (6 ) significance that the pulsar is undergoing long-term spin down at a rate˙¼ (À5:6 AE 2:0) ; 10 À16 Hz s À1 , with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1:5 ; 10 8 G for the pulsar's surface dipole magnetic field and Q/I < 5 ; 10 À9 for the fractional mass quadrupole moment. We also measured an orbital period derivative ofṖ orb ¼ (3:5 AE 0:2) ; 10 À12 s s À1 . This surprisingly largė P orb is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called black widow millisecond radio pulsars, which further strengthens previous speculation that SAX J1808.4À3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0:15 00 ) source position from optical data.

We present radio, X-ray, and γ-ray observations which reveal that the binary millisecond pulsar / low-mass X-ray binary transition system PSR J1023+0038 has undergone a transformation in state. Whereas until recently the system harbored a bright millisecond radio pulsar, the radio pulsations at frequencies between 300 to 5000 MHz have now become undetectable. Concurrent with this radio disappearance, the γ-ray flux of the system has quintupled. We conclude that, though the radio pulsar is currently not detectable, the pulsar mechanism is still active and the pulsar wind, as well as a newly formed accretion disk, are together providing the necessary conditions to create the γ-ray increase. The system is the first example of a transient, compact, low-mass γ-ray binary and will continue to provide an exceptional test bed for better understanding the formation of millisecond pulsars as well as accretion onto neutron stars in general.

One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. The flux we observe from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star's rotation, and this periodic modulation at the spin frequency is called a pulsation. As the photons propagate through the curved space-time of the star, information about mass and radius is encoded into the shape of the waveform (pulse profile) via special and general relativistic effects. Using pulsations from known sources (which have hotspots that develop either during thermonuclear bursts or due to channelled accretion) it is possible to obtain tight constraints on mass and radius. The second technique involves characterising the spin distribution of accreting neutron stars. A large collecting area enables highly sensitive searches for weak or intermittent pulsations (which yield spin) from the many accreting neutron stars whose spin rates are not yet known. The most rapidly rotating stars provide a very clean constraint, since the limiting spin rate where the equatorial surface velocity is comparable to the local orbital velocity, at which mass-shedding occurs, is a function of mass and radius. However the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasiperiodic oscillations in X-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state, and large area X-ray timing instruments would provide much improved detection capability. We illustrate how these complementary X-ray timing techniques can be used to constrain the dense matter equation of state, and discuss the results that might be expected from a 10m 2 instrument. We also discuss how the results from such a facility would compare to other astronomical investigations of neutron star properties.

Millisecond radio pulsars acquire their rapid rotation rates through mass and angular momentum transfer in a low-mass X-ray binary system. Recent studies of PSR J1824−2452I and PSR J1023+0038 have observationally demonstrated this link, and they have also shown that such systems can repeatedly transition back-and-forth between the radio millisecond pulsar and low-mass X-ray binary states. This also suggests that a fraction of such systems are not newly born radio millisecond pulsars but are rather suspended in a back-and-forth state switching phase, perhaps for giga-years. XSS J12270−4859 has been previously suggested to be a low-mass X-ray binary, and until recently the only such system to be seen at MeV-GeV energies. We present radio, optical and X-ray observations that offer compelling evidence that XSS J12270−4859 is a low-mass X-ray binary which transitioned to a radio millisecond pulsar state between 2012 November 14 and 2012 December 21. Though radio pulsations remain to be detected, we use optical and X-ray photometry/spectroscopy to show that the system has undergone a sudden dimming and no longer shows evidence for an accretion disk. The optical observations constrain the orbital period to 6.913 ± 0.002 hr.